By Mimi Stockton
Tires: It's not a sexy topic, but a very important one nonetheless. One so important that it deserves not one, but two articles! This week we talk tire types and tire pressures. Next week we discuss the pros and cons of a tubeless setup. Hopefully after reading these primers, you will feel more confident to equip your bike with the best, fastest and most practical tires available.
As we all know, XTERRA racing involves cross-country mountain biking and that requires tires that roll fast but grip well and have little resistance while climbing. They need to be lightweight and slender to help keep overall bicycle volume down, and they need to withstand the rigors of the XTERRA courses. Riders must consider tire width, knobs, rim width, intended use, price, rubber durability, sidewall protection, and a whole host of buzzwords unique to each tire manufacturer. And all this before you even think about how much air to put in them! There's no doubt tires are confusing. However, few buying choices have more impact on, or can better improve, your ride than tires—they are literally where tire meets trail—so giddy up and let’s get after it!
What kind of tires should I choose?
Your standard mountain bike tire has been designed to perform well in a variety of conditions, from hard pack to soft dirt, which means that it is all about compromises. However, that's not necessarily a bad thing as the very large majority of riders can't be bothered to swap their tires depending on what sort of shape their trails are in. But sometimes these tires, that have been designed with low rolling resistance and all around riding in mind, are not the best choice for an XTERRA race. Sometimes what's needed is a more specific tire. The fact is, nothing makes your bike faster than putting faster rolling tires on it, or a tire that is perfectly suited for the trail at hand. With all the choices out there, how do I pick the best ones?
One of the first things to figure out is what kind of rider you are. Do you ride XC, where light weight and rolling speed are priority? Or are you more focused on downhill, where grip and strength are what you need most? Since we are talking XTERRA here, let's assume we are all XC riders and not downhill speed demons.
Next, you want to match up a width and knob design for XC riding. You'll want something with short, fast knobs or lower-profile knobs in the center for faster rolling.
Now, consider different tread patters based on the terrain you most often ride. This however is where it gets tricky for us XTERRA racers because many of us race across the country and sometimes across the world. Each course is unique and can present us with vastly different terrain. One race you're cruising on hard-packed singletrack and another you're battling loose gravel, mud and sand. Having two sets of tires may be the way to go. But tread pattern is an area that you may want to investigate because it is really magic; how the knobs are spaced, whether there are 'transition' knobs between center and edge knobs, and how they are shaped and ramped all impact how a tire rides and performs.
Sidewall protection: how much do you need? If you find yourself riding a lot of rocky trails, you might want more protection. But keep in mind, the more protection you get, the more weight you carry. It's a trade-off. Light and tough--that's ideal. XTERRA courses can be long so you need to be efficient, but they can also be full of rocks and other obstacles that require some protection.
What about rubber compound? This is getting a bit technical, but the general rule of thumb is the softer the rubber the better it grips and but the faster it wears. And also, the more varied the rubber compounds on any given tire, the more expensive the tire. There are single, double and triple compound tires. Single are the hardest and cheapest while triple usually have center knobs that are hard, cornering knobs that are soft, and hard base layer to optimize rolling resistance. Generally, these tires are the most expensive option. In the middle are double compound tires which offer the most versatility--softer on sides, harder in the middle.
Along with the compound, you also want to figure out how thick your casing needs to be. Casing is the protective layer that runs bead-to-bead and gives a tire its flex characteristics. Often, tires are designed with just the right casing for their intended use, making the choice simple. Each casing type is tuned for its specific use, ensuring the best performance.
Finally, buy the right size tires for your rims. Tires are designed around rim widths. So even once you've figured out the riding style, knob profile, sidewall, rubber and casing, keep in mind that you shouldn't go out and run a very wide tire on a narrow rim, or vice versa. Generally, if you're running a rim wider than 30 mm, you'll want a wider tire (2.3-2.5"). If you're running a narrow rim–something closer to 20 mm–you'll want a narrower tire (1.9-2.2"), even though getting a wide one might be tempting. All tires are usually offered in a variety of widths. But buying the right size for your rim will ensure the knob profile will perform as designed and not be too 'round' (wide tire on a narrow rim) or 'flat' (narrow tire on a fat rim).
What's the takeaway? Every rider is different. And every trail is different. It pays to play around a bit with tires to really dial in what works for you on the majority of trails you ride. For example, do you want plenty of front end grip and less back end bite? Do you ride more centered, more front or off the back? Do you like tires that break evenly and predictably? Maybe having the same tire front and back is your jam. It's okay to have different front and back tires--just don't vary them too much. Make sure to answer the above questions before you plunk down money for your next set of rubber. And perhaps most importantly, buy a good tire gauge and inflate your new tires properly, because if you don't do that, all this is for naught!
Now, what about tire pressure?
Tire pressure is a critical component in getting the most out of your mountain bike. There are a number of variables that go into figuring out the ideal pressure range for a tire. So what’s the magic bullet pressure, one that will make you shred harder than the pros? Unfortunately, tire pressure is as personal as the fit of our saddles or what beer we choose at the bar, and depends on weight, riding style, tire choice and the trails we ride! But there are a few things to think about when pumping up those tires.
There are certain pressures that will generally work well for most people in most situations, but if you want to maximize your riding enjoyment and unlock extra grip, there is a better way to find your optimum setup that works for you. When it comes to finding the perfect pressure, it’s all about finding that balance between grip and stability. What happens if you run too high? Higher tire pressures help support the sidewall of the tire offering increased stability and increased protection for the rim, but go too far and traction will be drastically reduced as the contact patch shrinks and the ride will feel harsh. What if I go too low? Low pressures increase grip from the larger contact patch and improve cornering traction as the softer tire can wrap around trail imperfections. However run the tires too low and you drastically increase the risk of rim damage, and the softer air pressure reduces the natural spring of the tire which can create a wobbly and unstable ride at speed. In hard turns the tire tends to lack stability in the sidewall and can feel shifty.
Keep in mind these variables when figuring out your ideal pressure--they all impact tire performance:
1. How much do you weigh? How a tire performs at a given pressure is relative to the rider’s weight. For example, a 150 lb. rider might find that 28 PSI in a tire feels too hard and lacks traction, whereas a 225 lb. rider might find this pressure too low resulting in a squirmy tire.
2. What is the terrain like where you mostly ride (and race)? Fast and flowy? Or a bunch of chunky rock gardens? If the latter, consider bumping up your pressure to combat pinch flats and sidewall damage.
3. What's your riding style? How you ride is as important as where you ride. The more aggressive your style, the more likely it is that you'll need to run a bit more pressure. Or are you a finesse type – one that rides 'gently" on the bike? Do you try to pick the cleanest line through a rock garden, or do you prefer to charge ahead with reckless abandon? Do you keep your wheels on the ground, or do you enjoy launching off jumps and drops?
4. What is your tire's volume? Tire volume and pressure go hand in hand. A tire’s volume will determine how a given pressure 'feels'. For example, a certain psi in a 29 x 2.25 tire might feel almost flat, while this same psi in a 26 x 2.0 model will feel rock solid to the touch – the wheel will bounce off, rather than absorb, a trail's irregularities.
5. How wide are your rims? This plays a critical role in determining how low you can go without sacrificing performance. A wide rim does a better job of supporting a tire than a narrow one. For a given tire size, a wider rim will allow you to run a lower pressure without the tire squirming and folding underneath you.
6. Finally, what's the construction of your tire? The way a tire's casing is made will impact how it feels at a given pressure in much the same way as your weight does. Tires with high TPI (threads per inch) casings are generally more supple than those with low TPI counts.
Sorting out a tire pressure that's right for you is as much an art as a science. Keep these six factors in mind and take the time to experiment. The best way to figure out your ideal tire pressure is to hit the trails. Arm yourself with a pump and gauge and find a short one to two minute test loop with some nice flat corners, berms, rocks and roots--representative of the terrain you ride but nothing too crazy; you want to be able to concentrate on "feel" while you ride, not worry about crashing into a tree. Think about how much grip you have, how "hard" or "soft" the ride feels, especially when going over roots and rocks. Repeat this exercise several times, lowering or raising the pressures until you find that optimal balance between performance and stability. You will probably find that you are running a slightly different pressure in the front than in the back.
Play around with your pressures BEFORE a race--not the morning of. You want to have enough knowledge of what works for you to be able to adjust to race day conditions. If it's been raining for the past couple of days before your race, turning the nice hard packed trails into a mud festival, you should know how to adjust accordingly (either put on mud tires and increase the Psi or If using regular tires, drop a few Psi). Don't rely on other people to tell you what pressures to run. Everyone is different and everyone has different magic numbers!
The XTERRA Couch to XTERRA training series is presented by SheriAnne Little, Jeffrey Kline, and four-time XTERRA age group world champion Mimi Stockton of PRS Fit. Their new 12-week “Couch-to-XTERRA” training program is designed to do just that, get aspiring athletes off the couch, into training, and to the start line of an XTERRA. Read past training articles from PRS Fit at http://www.xterraplanet.com/training/couch-to-trail and learn more about their coaching programs at prsfit.com.